The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to an improved fast spin echo pulse sequence for use in diffusion weighted MR imaging.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant y of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the xy plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field Bo, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging ("EPI") are well-known, and a number of variations on this pulse sequence are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,716,369; 4,355,282; 4,588,948 and 4,752,735. Unfortunately, even when state-of-the-art fast gradient systems are used, the EPI pulse sequence has difficulties with eddy current dependent and susceptibility induced image distortions.
A variant of the echo planar imaging method is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled "RARE Imaging: A Fast Imaging Method for Clinical MR." The essential difference between the RARE sequence and the EPI sequence lies in the manner in which echo signals are produced. The RARE sequence utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill sequence, while EPI methods employ gradient recalled echoes. This "fast spin echo" pulse sequence is generally considered a problem free technique for acquiring multiple k-space lines with one excitation. It is, for instance, much less sensitive to field inhomogeneities and gradient timing errors than echo planar imaging. Further, because the readout gradient is always positive, unlike echo planar imaging, gradient fidelity is less of a problem.
However there are several critical parameters with the fast spin echo pulse sequence, which, if set incorrectly, can produce considerable image artifacts. These involve the radio frequency (RF) pulse spacing and phase relationships, and also the areas of the readout gradient pulses. Firstly, it is necessary that the time between the centers of the excitation pulse and first refocusing pulse should be half the time between the centers of adjacent refocusing pulses. Secondly, the echoes and the refocusing RF pulses should have the same phase angle. This is usually achieved by setting the phase of the excitation RF pulse to 90.degree. with respect to the phase of the refocusing RF pulses. Related to these requirements is the fact that the area of the readout gradient pulse between the excitation and the first refocusing RF pulse should be equal to half the area of the readout gradient pulse between each of the subsequent refocusing pulses.
For conventional fast spin echo ("FSE") imaging the above critical parameters can be controlled in a relatively straight forward manner. However, there are a number of imaging situations where the required degree of phase control between the rf pulses and the echoes is difficult to achieve. One of these situations is diffusion weighted imaging, where large gradient pulses are employed and resulting eddy currents are more prevalent.
Diffusion weighted imaging employs a pair of large gradient pulses at the beginning of the pulse sequence to sensitize the acquired NMR signals to spin motion. Generally, such imaging is performed using a single shot EPI pulse sequence, however, severe image distortion can occur. One solution as proposed by Butts, et al., in Magn. Reson. Med., 38, 741-749 (1997) is to use a multi-shot EPI acquisition with navigator signals used to correct for phase errors.
Diffusion weighted FSE has been attempted, but the large motion encoding gradients at the beginning of the pulse sequence set up Eddy currents which interfere with the phase relationship between the excitation and refocusing pulses. Norris, et al. Proposed in Magn. Reson. Med., 27, 142-164 (1992) a method for controlling the phase in an FSE pulse sequence which involved separating out two coherence pathways, and using only one of the coherence signals. One problem with this approach is the strong oscillation of the echo signal amplitude which, if uncorrected, causes severe ghosting in the image. A similar idea has been proposed by Shick in Magn. Reson. Med., 38, 638-644 (19997) in which the echo signal amplitude is increased. Alsop discloses in Magn. Reson. Med., 38, 527-533 (1997) a method for reducing the oscillations in the amplitude of these echo signals.